Static power converters utilizing wave synthesis techniques have proved advantageous in saving space and weight when low frequency signal waveforms are required. This is due in part to the savings in the size and weight of magnetic components permitted by the high frequency of the carrier signal. The typical waveform synthesizer pulse width modulates a high frequency switched waveform and filters the modulated pulses to obtain the synthesized waveform. The switched waveform may have two levels or states of magnitude (positive and negative) or it may have three levels (positive, negative and zero).
A common technique is to use a switched waveform whose frequency (or period) is constant and to modulate the percentage of the period spent at one level, such that the average value of the waveform during a particular period equals the value of the synthesized waveform (normally a sinewave) at that same point in time.
It has been found that the use of a constant carrier frequency for a two level switching waveform does not permit optimum suppression of low order harmonics of the synthesized waveform. The harmonic content of the pulse width modulated constant frequency carrier tends to be concentrated at the carrier frequency and at multiples thereof. Hence, filtering is much more difficult than if this harmonic content is spread over a range of frequencies.